Metal matrix composites (MMCs) are metals or alloys strengthened with tiny inclusions of another material which inhibit crack growth, and increase performance. MMCs have mechanical properties that are superior to those of most pure metals, some alloys, and most polymer-matrix composites, especially at high temperatures. The ability to tailor both mechanical and physical characteristics of MMCs is a unique and important feature of these materials.
Although the technology is relatively young, there are a number of significant applications, most notably, the space shuttle fuselage struts, space telescope boom-waveguides, and diesel engine pistons. In the future, metal-matrix composites are expected to become an important class of materials in numerous other commercial applications.
Although many metal-matrix composites having widely different properties exist, some general advantages of these materials over competing materials can be cited. MMCs are known to have higher strength-to-density ratios and higher stiffness-to-density ratios with better fatigue resistance than most unreinforced metals and some polymer matrix composites.
Numerous combinations of matrixes and reinforcements have been attempted since work on metal matrix composites began in the late 1950's. The most important matrix materials have been aluminum, titanium, magnesium, copper, and superalloys. Particular metal matrix composites that have been employed in the art have included aluminum matrixes containing boron, silicon carbide, alumina, or graphite in continuous fiber, discontinuous fiber, whisker, or particulate form. Magnesium, titanium, and copper have also been used as matrix metals with similar ceramic inclusions. Additionally, superalloy matrixes have been impregnated with tungsten wires to provide greater creep resistance at extremely high temperatures, such as those found in jet turbine engines.
Fabrication methods are an important part of the design process for MMCs. Considerable work is underway in this critical area, and significant improvements in existing processes appear likely. Current methods can be divided into two major categories: primary and secondary fabrication methods. Primary fabrication methods are used to create the metal matrix composite from its constituents. The resulting material may be in the form that is close to the desired final configuration, or it may require considerable additional processing, called secondary fabrication. Some of the more popular secondary fabrication methods include forming, rolling, metallurgical bonding, and machining.
One of the more successful techniques for producing MMCs, first suggested by Toyota for making pistons in 1983, is by infiltrating liquid metal into a fabric or prearranged fibrous configuration called a preform. Frequently, ceramic and/or organic binder materials are used to hold the fibers in position. The organic materials are then burned off before or during metal infiltration, which can be conducted under a vacuum, positive pressure, or both. One commonly employed pressure infiltration technique, which is known to reduce porosity in the final composite, is referred to as squeeze-casting.
The squeeze-casting process usually consists of placing a fiber or whisker preform in a cavity of a die, adding molten metal, and infiltrating the preform with the metal by closing the die and applying high pressure with a piston. The process is typically used for near net shaped parts of small dimensions. See Siba P. Ray and David I Yun, "Squeeze-Cast Al.sub.2 O.sub.3 /Al Ceramic-Metal Composites," Ceramic Bulletin, Vol. 70, No. 2 (1991).
Although Ray and Yun suggest that ceramic matrix composites can be manufactured using preforms composed of alumina particles of 0.2 micron average particle size and including 14 to 48% open pores, this disclosure is limited to the production of ceramic-matrix composites (CMCs) having severely limited toughness, ductility, and machinability. Their set-up requires the use of expensive, heavy-walled dies and presses designed to withstand large pressure differentials, such as a 1,500-ton press.
It is also known to produce metal matrix composites by squeeze-casting followed by a secondary fabrication procedure, as suggested by Nishida et al., U.S. Pat. No. 4,587,707. In this process, squeeze-casting is used to infiltrate a porous shaped article of ceramic particles with a molten metal, which is then permitted to solidify. High pressures of 500 to 1,000 atmospheres (15,000 to 150,000 psi) were believed to be required for complete infiltration. The ceramic particles are provided by slender rods and are not uniformly distributed in the matrix. Since these concentrated layers of ceramic in the metal matrix are not intended to be present in the final product, mechanical forming is used to break up the rods into smaller pieces and distribute them throughout the matrix. The suggested rolling or extrusion techniques help to spread the now broken ceramic preforms more randomly throughout the composite; however, the result is far from a uniform distribution on a microscopic scale. Since the sintered ceramic rods are likely to be fractured in a non-uniform manner during the mechanical forming step, the resulting composite may contain concentrated, or agglomerated ceramic regions, which could limit the resulting composite's properties.
To alleviate the need for large pressure requirements, most known metal infiltration procedures use large particulate ceramics, greater than about 1 micron. Molten metal infiltration has not been considered a practical process for making metal-matrix composites incorporating submicron ceramic particles because the press size and pressure needs would be excessive and unrealistic. See Christodoulou et al., U.S. Pat. No. 4,916,030, Col. 2, lines 25-38.
In order to dispense with the limitations and expense of large multi-ton presses, others have employed inert gas pressure metal infiltration techniques with loose ceramic powders. See Jingyu Yang and D. D. L. Chung, "Casting Particulate and Fibrous Metal-Matrix Composites by Vacuum Infiltration of a Liquid Metal Under an Inert Gas Pressure," Journal of Materials Science, Vol. 24, p.p. 3605-3612 (1989). Yang and Chung have developed a low pressure (1,000 to 2,500 psi) molten metal infiltration technique that employs pressurized inert gas for forcing molten metal into loose ceramic fibers or particles. Particles ranging in size from 0.05 to 5 microns are used. By limiting the particles to a specific size range, this reference teaches that greater porosity in the close-packed particles can be provided, since the gaps between the particles are not filled by significantly smaller particles. It is this porosity volume fraction that is relied upon to permit the low pressure force to cause the molten liquid to infiltrate the loose layers of ceramic particles. Unfortunately, since the particles are loose and not sintered, they tend to agglomerate and randomly orient themselves during metal infiltration. This results in a relatively non-uniform distribution of particles throughout the matrix. Despite the expedient of using less pressure, therefore, the composite produced by infiltrating loose particles fails to achieve its full ductility and strength.
Metal-matrix composites are not without other well-recognized drawbacks. The ceramic inclusions used to strengthen these composites are extremely hard, and are difficult to machine using conventional techniques. This results in serious tool-wear problems when the composite is machined into its final configuration. In some cases, the tool-wear becomes such a serious problem, that manufacturers resort to near-net shape manufacturing techniques, such as die casting and squeeze-casting, and the like, where machining is kept to a minimum, or is eliminated altogether. As reported in Charles T. Lane's "Machining Characteristics of Particulate-Reinforced Aluminum," Fabrication of Particulates Reinforced Metal Composites, Proceedings of an International Conference, Montreal, Quebec, Canada, ASM International, pp. 195-201 (1990), aluminum alloys reinforced with 10 to 15 micron ceramic particles wore through high-speed steel (HSS) tools in a matter of seconds, and dulled conventional and coated carbides in a matter of a few minutes. This paper reported that the only cost-efficient machining technique for MMCs was to use polycrystalline diamond (PCD) tools at speeds of up to 2,438 meters per minute. Other artisans have had similar experiences with machining MMCs, which has obviously limited their full commercial implementation.
Accordingly, there is a need for further process developments for manufacturing metal-matrix composites which have superior strength and uniformity, but which are also easy to machine and manufacture. There also remains a need for economically producing metal-ceramic composites without expensive heavy press machinery, or complicated processing techniques.